A solution of urea and ammonium nitrate in water (known as UAN) is commonly used as a fertiliser, supplying nitrogen to plants. The solution may be prepared by mixing urea (as a solid or solution) with ammonium nitrate (as a solid or solution) and process water. The pH of the product may be adjusted by addition of nitric acid or ammonia, and a corrosion inhibitor may be added. Many soils also require sulphur as a plant nutrient.
CA 811080 proposes preparing stable suspensions of finely divided sulphur in a concentrated aqueous solution of urea and ammonium nitrate, utilizing a clay such as attapulgite as stabiliser, by applying shear to a pregelled mixture of clay, urea, ammonium nitrate and water, and subsequently adding the finely divided sulphur. The amount of finely divided sulphur, for example with 98% of particles able to pass through a 100 mesh (which is approximately 150 microns), in the suspension is variable and ranges between about 10% to about 50% by weight, preferably between about 15% to about 30% by weight. U.S. Pat. No. 3,519,413 also looks to prepare stable suspensions of fertilisers, which in one combination may be UAN fertilisers. U.S. Pat. No. 3,519,413 discloses that finely divided particles in the context of a fertiliser for application to the soil are particularly particles with 90% of the solids being of a size above 0.1 mm, which is approximately 100 microns.
U.S. Pat. No. 5,082,487 discloses solutions of ammonium sulphate, ammonium nitrate and urea, which can be used to deliver both nitrogen and sulphur to plants. The solutions comprise at least about 12.5 percent by weight of ammonium sulphate (which is equivalent to at least about 3 percent by weight of elemental sulphur). Such a fertiliser solution provides a soluble form of sulphur and this may leach when applied to the soil. An alternative means of applying sulphur is to supply a mixture of UAN with ammonium thiosulphate. The ammonium thiosulphate is a relatively expensive product.
The present inventors have sought to provide an alternative fertiliser that can provide both nitrogen and sulphur to the soil, and that avoids some of the problems of the known fertilisers.
The present inventors have also sought to provide a use for weak aqueous solutions of urea and ammonia (known as scrubber liquor) that are produced as a waste product of the urea manufacturing process. The streams are produced as a byproduct of gas treatment processes that are employed to reduce dust. The scrubber liquor may additionally comprise salts such as ammonium sulphate or nitrate if ammonia emissions are also controlled. Treatment of the scrubber liquor streams is costly and involves the evaporation of water from such streams or the production of UAN solutions. It would be advantageous to devise a process in which scrubber liquors are a useful feedstock.